Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique that is used to diagnose many types of injuries and medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The main magnetic field causes an alignment of the magnetic moments of the nuclei within the body to be aligned with a net result in a parallel orientation. The main magnetic field causes the protons to rotate around the equilibrium axis with a frequency, known as the Larmor frequency, that is characteristic for the nuclei to be imaged. An external radiofrequency (RF) field applied by other hardware within the MRI system perturbs the magnetization from its equilibrium state.
Upon termination of the application of the RF pulse, the magnetization is relaxed to its original position. The exponential time rate at which the nuclei to relax to its original position is often referred to as the spin-lattice relaxation time, or T1; the exponential rate at which the nuclei mutually diphase is often referred to as the spin-spin relaxation time, or T2. During relaxation the rotating magnetic moment induces a time varying voltage in the receive coil that can be detected. An image processor then reconstructs an image representation from the received magnetic resonance signals for display on a human readable display.
Magnetic resonance elastography (MRE) is a technique that combines the use of a mechanical actuator agitating the tissue of the body with MR imaging. A mechanical actuator creates waves between 100 to 1000 Hz within the tissue of interest, and imaging by the MR imager is gated or otherwise synchronized to that frequency, as MR images are motion-sensitive. The differing shear modulus, or softness, of each type of tissue promotes a different wave pattern within the tissue caused by the agitation of the mechanical waves. The technique promotes detection of subtle changes in tissue displacement and determines the mechanical tissue properties quantitatively. This is especially beneficial in the diagnosis of disorders that have a characteristic stiffness of a region of tissue with respect to the surrounding tissue, and is important in the diagnosis of many thyroid, prostate, breast and abdominal pathologies.
Ultrasound energy is commonly used in ultrasound imaging, which is alternatively known as ultrasound scanning or sonography. Ultrasound imaging is another type of medical imaging, and uses high-frequency sound waves to obtain images of the human body. The high-frequency sound waves are directed into the human body using an ultrasound transducer. The injected sound waves echo from the body's tissues and fluids, reflecting back into the probe, which is commonly a crystal attached to electrodes. The strength and characteristics of the reflected waves are recorded and the ultrasound system calculates the distance of the tissue from the probe and displays a real-time image of the tissues and fluids.
Ultrasound has many limitations, including difficulty of the ultrasound waves penetrating bone and gassy areas of the body containing air. Thus, internal characteristics of bone, the stomach and the intestines may not be properly imaged. Ultrasound waves rapidly loose strength as the distance of penetration increases. This may cause difficulties when imaging the deeper structures of the heart and abdomen areas, as well as difficulties in imaging obese patients. Also, the depth of ultrasound penetration rapidly decreases as the frequency of the waves is increased. However, ultrasound imaging can be quantitative, so as the small differences between two images taken at different depths of the tissue, wherein the second image is taken with the transducer applying more pressure to the tissue, can be used to establish the mechanical characteristics of the tissue.
Prior Art has combined the use of ultrasound with the use of MR. Examples can be seen in U.S. Pat. No. 5,247,935, U.S. Pat. No. 5,291,890, U.S. Pat. No. 6,735,461, and US Application Publication 2004/0267111. The past combination of ultrasound and MR has focused primarily on using the heat characteristics of the combined waves to destroy certain tissues within a patient's body, and using the combined methods for MR scan navigation. To date, no Prior Art has provided an effective method of using ultrasound waves to modify the MR signal characteristic primarily through altering the contrast of the MR images.